The present invention relates to an electrically conductive transmission line adapted for use in the magnet bore of a magnetic resonance system, and an invasive medical instrument comprising a transmission line and a magnetic resonance coil used for tracking or imaging and a transmission line.
A wide range of techniques have been proposed to track the position of interventional instruments in magnetic resonance. Among these, active tracking with device-borne miniature receive coils have proven to be very fast and comfortable with almost any imaging sequence. Also, interventional instruments with local imaging coils have been used for intravascular imaging.
Ackermann D. L. et al., Proc. of 5th SMRM, 1131 (1986) and C. L. Dumoulin et al., “Real time position monitoring of invasive devices using magnetic resonance”, Magn. Reson. Med. 29, 411-415 (1993) disclose for example a real time position monitoring of invasive devices using magnetic resonance. In this technique, projections of the entire patient, or in general of the entire object to be imaged, onto one spatial direction are acquired with such miniature coils.
Due to the local reception characteristics of the miniature coils the projections resemble signal only at the position of the coil. For a device with one tip coil, a simple maximum search along the projector reveals the respective coordinate of the coil. This scheme is typically repeated for projections along the x, y and z direction to obtain the 3D coordinates of the coil in only three sequence repetition times, usually in the order of 20 milliseconds.
The underlying principle of such kind of MR tip tracking is to provide a magnetic field which varies monotonically with the local position, which as a consequence also leads to a variation of the resonance frequency of a sample with the position. The miniature receive coil incorporated for example into an interventional device will pick up an RF signal with a locally specific RF frequency which thus permits to track the position and/or orientation of the device within the coordinate system defined by the magnetic field gradients of the MR imaging system.
However, cable connections to MR receive coils and active interventional devices can cause strong RF heating especially at the tip of such devices. This is known for example from Ladd M E, Quick H H, Boesiger P, McKinnon G C. RF heating of actively visualized catheters and guidewires. In: Proceedings of the ISMRM, 6th Scientific Meeting and Exhibition, Sydney, 1998. p 473, as well as from Konings M K, Bartels L W, Smits H F M and Bakker C J G 2000 Heating around intravascular guidewires by resonating RF waves J. Magn. Reson. Imaging 12 79-85 and Nitz W R, Oppelt A, Renz W et al. On the heating of linear conductive structures as guide wires and catheters in interventional MRI. J Magn Reson Imaging 2001; 13:105-114.
The reason for strong RF heating is due to resonances of the transmission line. If such a resonance occurs, the incident RF wave is bounced back at the end points of the wire-like structure which causes the reflected RF waves to travel back and forth along the longitudinal axis of the conductive structure in such a way, that standing RF waves are formed. These standing RF waves lead to a strong heat dissipation. In order to solve the problem of heat dissipation and thus the problem of standing RF waves, transformer based cables (safe transformer lines, STL) have been proposed that avoid such heating, for example in Schulz V, Gleich B.: Magnetic resonance imaging apparatus provided with an electrical accessory device. Priority 23 Oct. 2002 at German Patent Office as DE10249239.5. International Filing on 15 Oct. 2003 as PCT/IB03/04589; Weiss S, Vernickel P, Schaeffter T, Schulz V, Gleich B. Transmission Line for Improved RF Safety of Interventional Devices. Magn Reson Med 2005; 54:182-189; Vernickel P, Schulz V, Weiss S, Gleich B. A Safe Transmission Line for MRI. IEEE Trans BME 2005; 52(6):1094-1102.
A ‘transformer based cable’ consists of cable sections connected by ‘transformers’, i.e. inductive coupling elements, which block currents that would lead to RF heating. A schematic view of an electrically conductive transmission line 100 is shown in the view of FIG. 1, wherein the transmission line consists of inductive coupling elements 106 for coupling lead segments 104 of the transmission line. The end of the transmission line is terminated by a miniature coil 102.
A transmission line of this kind is known for example from WO 2006/003566 A1.
As further shown in the schematic views of FIGS. 2a and b, the transformers are realized as resonant single loop transformers to achieve high signal transmission and miniaturization. For example, a substrate 200 is used, like a printed circuit board (PCB) substrate on which the lead segments 104 are printed on top of each other. This leads to an inductive coupling between ‘neighboring’ lead segments of the transmission line 100. FIG. 2 is a top view of the electrically conductive transmission line, wherein FIG. 2b is a respective side view.
The transformer 106 comprises in-plane loops. A first and second loop with a cross section of about 25×25 μm are provided with a length of about 5 cm. The lateral distance in FIG. 2a is about 500 μm and the horizontal distance in FIG. 2b is about 127 μm. The first and second loop couple inductively.
It is a goal of the invention to provide an improved electrically conductive transmission line and an improved invasive medical instrument.